The objective of this exploratory and developmental (R21) project is to determine the role of macrophage migration inhibitory factor (MIF) in the development of osteoarthritis (OA) in mice and obtain data on MIF function in human joint tissues. The proposal is supported by an exciting preliminary study in which we discovered that 12 month-old MIF knock-out mice develop significantly less spontaneous OA than age and strain-matched controls, implicating MIF as a key cytokine involved in the development of age-associated OA. Originally discovered as a factor produced by T cells that inhibited macrophage migration, MIF was subsequently found to be produced by many cell types, including macrophages and synovial fibroblasts. MIF has been noted to function in the regulation of innate and adaptive immune responses and has been implicated in a number of acute and chronic inflammatory conditions including sepsis, atherosclerosis, and rheumatoid arthritis. Important to the novelty of this proposal, a role for MIF in the pathogenesis of OA has never been studied. In aim 1, we propose to determine if deletion of MIF reduces the severity of spontaneous age-related OA and injury-induced OA in mice. We hypothesize that MIF-/- mice will have less injury-induced and age-related OA compared to age- and strain-matched controls. Studies on MIF-/- mice will be complemented by studies in wild type mice using a monoclonal antibody that inhibits MIF activity. In aim 2, we will determine the mechanism for MIF activation of catabolic pathways in joint tissue cells. We hypothesize that MIF will be released from synovial fibroblasts, chondrocytes, and meniscal cells in response to catabolic stimuli and act in an autocrine and paracrine manner via its receptor, CD74, to promote the production of catabolic mediators. Experiments in this aim will include measuring the severity of age-associated OA in CD74-/- mice. Currently, no treatment exists which can slow the progression of OA. Because antibodies and small molecule inhibitors have already been produced to inhibit MIF, the results of our work would be used to support further studies on MIF in OA and could be rapidly translated into a novel therapeutic approach. The elucidation of mechanisms relevant to MIF's role in joint tissue destruction in aim 2 will also be relevant to RA where similar catabolic pathways are active.